Okay, the more I talk with you the more I'm getting my head around it. Tell me if I'm getting this right here...The full siphon obviously is the first in line. The emergency upturned 90 is second and the water if it rose due to the full siphon being blocked would hit that opening first. Then lastly the open line, although the down turned 90 would be at the same depth as the full siphon line, the end of the air line is placed slightly higher than the emergency opening and should only turn into a full siphon if both of the others are blocked?